1. Field of the Invention
This invention relates to a process for reacting dimethylamine with an alcohol or mixture of alcohols to obtain an alkyldimethylamine while substantially decreasing the formation of undesired byproduct alkylmonomethylamine which comprises passing dimethylamine and said alcohol through a fixed bed containing copper, and optionally chromium oxide, mounted on magnesium aluminate spinel.
2. Description of the Prior Art
The reaction of fatty alcohols with dimethylamine, to yield the corresponding alkyldimethylamines has been known for many years. The reaction has been carried out in the liquid and vapor phase, at atmospheric or higher pressures, generally over a copper- or nickel-containing catalyst in the presence of hydrogen. Both stirred-autoclave and fixed-bed type reactors have been employed. In U.S. Pat. No. 3,223,734 dodecyl alcohol was reacted with dimethylamine in the presence of Raney Nickel to yield dodecyldimethylamine. The tertiary amine assay was only 69.5 percent. In U.S. Pat. No. 3,366,687 dimethylamine was reacted with various alcohols in the liquid phase in the presence of hydrogen over a fixed-bed of barium-copper-chromite catalyst. Yields of alkyldimethylamine were less than 75 percent.
More recently, German applications Nos. 2,749,064; 2,749,065 and 2,749,066 disclosed the use of copper on an alumina support promoted with rhenium, molybdenum, tungsten, zinc and chromium oxides. The reported yields of dimethyldodecylamine were less than 85 percent. U.S. Pat. No. 4,138,437 discloses a process for carrying out the reaction in the liquid phase over a copper-chromium oxide catalyst wherein the liquid alcohol is reacted with a circulating gaseous mixture containing hydrogen and 1-20 percent by volume dimethylamine to obtain up to 93.5 percent yield of dodecyldimethylamine.
German application 2,709,864 claims a method for preparing tertiary aliphatic amines by reacting an alcohol and dimethylamine over a catalyst containing copper (II) oxide, an alkali and/or alkaline earth metal oxide, and/or chromium (III) oxide. In accordance with the method claimed, the reaction may be carried out in a fixed-bed or stirred-autoclave reactor and the weight ratio of copper (II) and chromium (III) oxides to alkali and alkaline earth metal oxide can range between 1 to 0.002 and 1 to 0.1. Example 1 of this application employs a catalyst containing 1.0 percent Na.sub.2 O, while the six other examples employ catalysts containing 0.1 percent Na.sub.2 O or none at all. In Example 1, the yield of dodecyldimethylamine was only 82 percent. The yield of alkyldimethylamines exceeded 90 percent in the other examples. Thus, there is nothing in the application to show that the use of more than 0.1 percent alkali is advantageous.
Similar work is reported in an article published by A. Baikar and W. Richarz in Ind. Eng. Chem. Prod. Res. Dev., 16, 261-265 (1977). The authors employ a catalyst containing CuO (25 percent), Cr.sub.2 O.sub.3 (1 percent), Na.sub.2 O (0.1 percent) SiO.sub.2 (70 per cent), and H.sub.2 O (about 4 percent). It is stated that in the vapor phase fixed-bed process optimal results are obtained with a dimethylamine to alcohol molar ratio of 5.5 and that a 92-96 percent yield of dodecydimethylamine is obtained. No mention is made of the formation of alkylmonomethylamines.
We have found upon repeating this experiment that the type of chromatographic column used by Baikar and Richarz does not separate dodecylmonomethylamine from dodecyldimethylamine. With a 10 percent carbowax 20 M+2 percent KOH column we have shown that three to five weight percent of the product identified by Baikar and Richarz as dodecyldimethylamine is actually alkylmonomethylamine.
This is a very significant finding from an industrial standpoint because the corresponding alkylmonomethyl- and alkyldimethylamines have very close boiling points and are difficult to separate by distillation. The presence of too much of the alkylmonomethylamine impurity can cause problems in down-stream processes utilizing the alkyldimethylamine product. For example, when the alkyldimethylamine is reacted with benzyl chloride to make an alkyldimethylbenzylchloride solution, the impurity will be converted to alkylmethyldibenzyl chloride altering the properties of the final product.